Anaerobic fermentations of hydrogen and carbon monoxide involve the contact of a gaseous substrate-containing feed with an aqueous fermentation broth containing microorganisms capable of generating oxygenated organic compounds such as ethanol, acetic acid, propanol and n-butanol. The bioconversion of carbon monoxide results in the production of oxygenated organic compound and carbon dioxide. The conversion of hydrogen involves the consumption of hydrogen and carbon dioxide, and this conversion is sometimes referred to as the H2/CO2 conversion or, as used herein, the hydrogen conversion.
Typically the substrate gas for carbon monoxide and hydrogen conversions is, or is derived from, a synthesis gas (syngas) from the gasification of carbonaceous materials, from the reforming of natural gas and/or biogas from anaerobic digestion or from off-gas streams of various industrial methods. The gas substrate contains carbon monoxide, hydrogen, and carbon dioxide and usually contains other components such as water vapor, nitrogen, methane, ammonia, hydrogen sulfide and the like. For the sake of convenience, the substrate gas is referred to herein as “syngas” even though it may only contain one of carbon monoxide and hydrogen and may not be derived by the gasification of carbonaceous materials.
These anaerobic fermentation processes are suitable for continuous processes. The syngas is passed into a bioreactor the aqueous fermentation broth for the bioconversion. Off gases can be removed from the bioreactor, and aqueous broth can be withdrawn from the bioreactor for recovery of the oxygenated organic compound at a rate sufficient to maintain steady-state operation. For the anaerobic fermentations to be commercially viable, economies of scale are required. Hence, commercial scale reactors, i.e., those with liquid capacities of at least 1 million, and more often at least about 5, say, 5 to 25, million, liters would be advantageous.
Continuous syngas fermentation processes typically produce co-produced oxygenated organic compounds in addition to the sought, product oxygenated organic compound. The co-produced oxygenated organic compounds can be co-metabolites that are not desired or intermediate metabolites in the bioproduction of the sought, product oxygenated organic compound. Also, co-produced oxygenated organic compounds can be produced by contaminating, or adventitious, microorganisms present in the aqueous fermentation broth. In some instances, these co-produced oxygenated organic compounds may be produced at rates, relative to the production rate of the sought, product oxygenated organic compound, that cause a build-up of the co-produced oxygenated organic compound in the aqueous broth. This build-up of the co-produced oxygenated organic compound is particularly untoward where the co-produced oxygenated organic compound reaches concentration levels that are inhibitory or toxic to the microorganisms used for the syngas fermentation. In some other instances, the co-produced oxygenated organic compound, when at sufficient concentrations, can adversely affect the metabolic pathways of certain microorganisms used for the bioconversion of syngas. For instance, where an alcohol is the sought, product oxygenated organic compound, with some microorganisms, the presence of certain concentrations of free carboxylic acids can induce a product distribution shift in which the microorganisms to generate a higher percentage of carboxylic acids. The exponentially increasing production of the acids leads to an increasing acidity in the fermentation broth causing an eventual loss of the microorganism being able to maintain cell membrane potential and loss of the population of microorganisms.
Although the fermentation broth could be discarded in the event that the concentration of the undesired organic compound becomes excessive, nutrients for the fermentation would also be lost. Additionally, for commercial-scale bioreactors, disposal of the large volume of aqueous broth in a bioreactor can be problematic depending upon the capacity of the waste water treatment system. Since a commercial-scale bioreactor may contain in excess of 1 million liters of aqueous broth, it is likely that the waste water from the bioreactor would have to be slowly discharged to the waste water treatment system to prevent exceeding capacity. Thus, the downtime of the affected bioreactor would be extended, resulting in a further loss of production. The amount of water lost could also be an economic loss.
In some instances, the undesired organic compound may be capable of being selectively removed such as by ion-exchange resins or membrane separations. These approaches may not provide suitable selectivity and are capital intensive yet may only be required sporadically or intermittently. But when needed, these unit operations must be able treat large quantities of fermentation broth in a short period of time. Moreover, they suffer from potential issues with fouling.
Accordingly, processes are sought for the removal of at least one undesired organic compound from an anaerobic fermentation broth that involve low capital expense yet can relatively quickly effect the reduction in concentration of the at least one undesired organic compound while retaining the fermentation broth anaerobic and retaining nutrients. Such desired processes should be capable of treating the large volumes of fermentation broth associated with commercial-scale bioreactors even on a sporadic or intermittent basis.